1. Field of Use
The present invention relates to electrical band-pass filters.
2. Prior Art
Electrical band-pass filters which exhibit a narrow passband centered on a preselected center frequency and sharp skirts have been known in the art for several decades. Good technical discussions of prior art electrical band-pass filter design theory and apparatus can be found in many standard texts, including: Reference Data for Radio Engineers, 6th edition, Howard W. Sams & Co., Inc., Indianapolis, Ind., 1975, Chapters 7-10; Single Sideband for the Radio Amateur, 5th edition, American Radio Relay League, Inc., Newington, Conn., 1970; The Radio Amateur's Handbook, 57th edition, American Radio Relay League, Inc., Newington, Conn., 1978.
Various approaches have been used to attempt to achieve the various passband characteristics needed in present-day sophisticated communications systems and the like: narrow passband response centered on a preselected center frequency; sharp skirts down to 50 dB or more; very low ripple in the passband; low inter-modulation distortion; high frequency stability with respect to temperature; low passband insertion loss; high isolation between the input and the output; etc.
One approach has been the high-frequency crystal band-pass filter. Piezoelectric quartz crystals are used as the filter elements because of their very high Q values. Examples of such filters can be found in the references listed above.
Such high-frequency crystal band-pass filters, while producing some of the desired passband characteristics, nevertheless, exhibit many major deficiencies. For example, fabrication of such filters is both complicated and costly because each of the crystals must be checked and often modified before being put into the filter assembly in order to obtain satisfactory performance. While a narrow passband response can typically be achieved at a 3 dB down point using only a few crystals, many crystals are required to produce sharp skirts down to 50 dB or more. It is also very difficult to remove ripple in the passband because each crystal has its own electrical pole. Low inter-modulation distortion is very difficult to achieve because frequency-sensitive elements, such as inductors and capacitors, must be used in the filter assembly. High-frequency stability with respect to temperature is difficult to achieve because many of the required components are temperature sensitive. The required use of several crystals and associated components to produce a filter having a sharp passband and fairly sharp skirts necessarily results in a higher passband insertion loss. High isolation between the input and the output of the filter is difficult to achieve because of the high degree of coupling between various filter components. These and other deficiencies limit the usefulness of prior art high frequency crystal band-pass filters.
The ever-expanding use and concomitant crowding of the electromagnetic spectrum has generated an enormous need for technically better and less expensive high-frequency band-pass filters. Another approach other than the high-frequency band-pass filter has been the active filter. Technical discussions of such active filters can be found in the references listed above. Active filters, however, also exhibit many deficiencies.